The main process parameters of high-frequency straight seam welded pipes include welding heat input, welding pressure, welding speed, opening angle size, the position and size of the induction coil, the position of the resistor, etc. These parameters have a great impact on improving the quality of high-frequency welded pipe products, production efficiency, and unit capacity. Matching various parameters can enable manufacturers to obtain considerable economic benefits.
1. Welding heat input
In high-frequency straight seam welded pipe welding, the welding power determines the amount of welding heat input. When the external conditions are constant and the input heat is insufficient, the edge of the heated strip cannot reach the welding temperature and still maintains a solid structure to form a cold Welding that won’t even fuse. Confusion caused by too small welding heat input
This lack of fusion during inspection usually manifests itself as a failed flattening test, bursting of the steel pipe during the hydrostatic test, or cracking of the weld during the straightening of the steel pipe, which is a serious defect. In addition, the welding heat input will also be affected by the quality of the edge of the strip. For example, if there are burrs on the edge of the strip, the burrs will cause sparks before entering the welding point of the squeeze roller, causing a loss of welding power and reducing the heat input. Small, resulting in a lack of fusion or cold welding. When the input heat is too high, the edge of the heated strip exceeds the welding temperature, resulting in overheating or even overburning. The weld will also crack after being stressed. Sometimes the molten metal will splash and form holes due to the breakdown of the weld. Blisters and holes are formed by excessive heat input. During inspection, these defects mainly manifest as failure in the 90° flattening test, failure in the impact test, and bursting or leakage of the steel pipe during the hydraulic test.
2. Welding pressure (diameter reduction)
Welding pressure is the main parameter of the welding process. After the strip edge is heated to the welding temperature, the metal atoms are combined under the extrusion force of the squeeze roller to form a weld. The size of the welding pressure affects the strength and toughness of the weld. If the applied welding pressure is too small, the welding edge cannot be fully fused, and the remaining metal oxides in the weld cannot be discharged and form inclusions, resulting in a greatly reduced tensile strength of the weld and the weld is prone to cracking after stress; If the applied welding pressure If it is too large, most of the metal reaching the welding temperature will be extruded, which not only reduces the strength and toughness of the weld but also produces defects such as excessive internal and external burrs or lap welding.
The welding pressure is generally measured and judged by the diameter reduction of the steel pipe before and after the extrusion roller and the size and shape of the burrs. Effect of welding extrusion force on burr shape. The amount of welding extrusion is too large, the spatter is large and the extruded molten metal is large, the burrs are large and tip over on both sides of the weld; the amount of extrusion is too small, there is almost no spatter, and the burrs are small and piled up; the amount of extrusion When it is moderate, the extruded burrs are upright, and the height is generally controlled at 2.5~3mm. If the welding extrusion amount is properly controlled, the metal streamline angle of the weld is symmetrical up, down, left, and right, with an angle of 55°~65°. The metal streamlines the shape of the weld when the amount of extrusion is properly controlled.
3. Welding speed
Welding speed is also the main parameter of the welding process. It is related to the heating system, weld deformation speed, and metal atom crystallization speed. For high-frequency welding, the welding quality increases as the welding speed increases. This is because the shortening of the heating time narrows the width of the edge heating zone and shortens the time for the formation of metal oxides. If the welding speed is reduced, not only the heating zone becomes wider, That is, the heat-affected zone of the weld becomes wider, and the width of the melting zone changes with the change of input heat, and the internal burr formed is also larger. Fusion line width at different welding speeds. During low-speed welding, the corresponding reduction in input heat will make welding difficult. At the same time, it is affected by the quality of the board edge and other external factors, such as the magnetism of the resistor, the size of the opening angle, etc., which can easily cause a series of defects. Therefore, during high-frequency welding, the fastest welding speed should be selected for production according to the specifications of the product as much as possible under the conditions allowed by the unit capacity and welding equipment.
4. Opening angle
The opening angle is also called the welding V angle, which refers to the angle between the strip edges in front of the extrusion roller, as shown in Figure 6. Usually, the opening angle varies between 3° and 6°, and the size of the opening angle is mainly determined by the position of the guide roller and the thickness of the guide sheet. The size of the V angle has a great impact on welding stability and welding quality. When the V angle is reduced, the distance between the strip edges will be reduced, thereby strengthening the proximity effect of high-frequency current, which can reduce the welding power or increase the welding speed and improve productivity. If the opening angle is too small, it will lead to premature welding, that is, the welding point will be squeezed and fused before reaching the temperature, which will easily form defects such as inclusions and cold welding in the weld, reducing the quality of the weld. Although increasing the V angle increases power consumption, under certain conditions it can ensure the stability of strip edge heating, reduce edge heat loss, and reduce the heat-affected zone. In actual production, to ensure the quality of the weld, the V angle is generally controlled at 4° to 5°.
5. Induction coil size and location
The induction coil is an important tool in high-frequency induction welding. Its size and position directly affect the efficiency of production.
The power transmitted by the induction coil to the steel pipe is proportional to the square of the gap on the surface of the steel pipe. If the gap is too large, the production efficiency will be sharply reduced. If the gap is too small, it will easily catch fire with the surface of the steel pipe or be damaged by the steel pipe. Usually, the inner surface of the induction coil is in contact with the pipe body. The gap is chosen to be around 10mm. The width of the induction coil is selected according to the outer diameter of the steel pipe. If the induction coil is too wide, its inductance will decrease, the voltage of the inductor will also decrease, and the output power will decrease; if the induction coil is too narrow, the output power will increase, but the active power loss of the tube back and the induction coil will also Increase. Generally, the width of the induction coil is 1 to 1.5D (D is the outer diameter of the steel pipe) which is more suitable.
The distance between the front end of the induction coil and the center of the squeeze roller is equal to or slightly larger than the pipe diameter, that is, 1 to 1.2D is more appropriate. If the distance is too large, the proximity effect of the opening angle will be reduced, causing the edge heating distance to be too long, making it impossible to obtain a higher welding temperature at the solder joint; if the distance is too small, the extrusion roller will generate higher induced heat, reducing its service life.
6. Function and location of resistor
The resistor magnet is used to reduce the flow of high-frequency current to the back of the steel pipe, and at the same time concentrate the current to heat the V-angle of the steel strip to ensure that the heat will not be lost due to the heating of the pipe body. If the cooling is not sufficient, the magnet bar will exceed its Curie temperature (about 300°C) and lose magnetism. Without the resistor, the current and induced heat would be dispersed around the entire pipe, increasing the welding power and causing the pipe to overheat. There is no thermal effect of the resistor in the tube blank. The placement of the resistor has a great impact on the welding speed, but also the welding quality. The practice has proved that when the front end of the resistor is exactly at the center line of the squeeze roller, the result will be flattening. When extending beyond the center line of the extrusion roller toward the side of the sizing machine, the flattening effect will be significantly reduced. When it is less than the center line but on one side of the guide roller, the welding strength will be reduced. The position is that the resistor is placed in the tube blank below the inductor, and its head coincides with the center line of the extrusion roller or is adjusted 20 to 40mm in the forming direction, which can increase the back impedance in the tube, reduce its circulating current loss, and reduce the welding power.
7. Conclusion
(1) Reasonable control of welding heat input can achieve higher weld quality.
(2) It is generally appropriate to control the extrusion amount at 2.5~3 mm. The extruded burrs are upright and the weld can obtain higher toughness and tensile strength.
(3) Control the welding V angle at 4° to 5°, and produce as high a welding speed as possible under the conditions allowed by the unit capacity and welding equipment, which can reduce the occurrence of some defects and obtain good welding quality.
(4) The width of the induction coil is 1 to 1.5D of the outer diameter of the steel pipe, and the distance from the center of the extrusion roller is 1 to 1.2D, which can effectively improve production efficiency.
(5) Ensure that the front end of the resistor is exactly at the center line of the squeeze roller to obtain higher tensile strength of the weld and good flattening effect.
Post time: Apr-23-2024